United States photovoltaic (PV) generation capacity has increased about 30% annually since 2000, with most of the increase resulting from residential installations. Production of PV cells is typically highly energy intensive, requiring substantial amounts of high-grade silicon or other rare materials, with an energy payback time on the order of 2-3 years. Concentrating PV (CPV) is an elegant means to intensify the power density of the solar radiation and deliver it to a smaller PV cell.
In CPV systems, refractive or reflective optical components such as lenses or mirrors, respectively, are used to concentrate sunlight onto a high-performance (high efficiency) photovoltaic solar cell. By concentrating sunlight, the amount of energy incident on the solar cell per unit area is increased compared to unaltered sunlight and the electrical energy per unit of solar cell area is commensurately increased. Thus, CPV allows for generating more electricity from a solar cell with a given surface area than by simply exposing that cell to direct, unaltered sunlight.
CPV systems have advantages over the traditional fixed flat-panel photovoltaic (FFP or PV) systems that are presently widely used. First, CPV systems concentrate sunlight onto a small, high-performance, photovoltaic cell, usually a multi junction cell (MJC); this reduces the amount of semiconductor needed by a factor of 100-1000 compared to flat-panel systems. Second, multi junction cells are more efficient at converting sunlight into electricity (˜36%) than traditional silicon semiconductor material used for flat-panel PV (˜14-19%). Third, CPV systems typically use dual-axis solar tracking, which enhances energy production by about 30% compared to FFP. Overall, CPV systems often extract about 30-45% more power than comparably rated, comparably priced FFP systems.
There are two principal means by which concentration is achieved in CPV systems that are currently available or in development (FIG. 1). The first category of concentrators uses lenses, reflectors, and other optics to channel and precisely focus incoming solar radiation onto a target MJC cell. This type of focusing typically requires that the concentrated sunlight be fairly homogeneous in intensity over the area of the MJC cell. This can require that optics be mechanically oriented with a specific attitude toward the sun.
Typically, a square or rectangular array of cells is mounted in a regular pattern so that all the cells are oriented in the same direction. Concentrating optics are mounted in front of each cell. The entire array is rotated, slowly and continuously, by mechanical means to maintain the solar rays at right angles to the concentrating optics (FIG. 1, left). The sun moves from east to west over the course of a day. Because of the curvature of the Earth, the sun follows an arc in the north south direction as well, both over the course of a day and also more slowly as the seasons change. Thus, the CPV array must usually be capable of tracking the sun in two directions (east-west) and (up-down or north-south). This is why these systems are called dual-axis tracking arrays.
The second broad category of CPV systems, luminescent concentrators (FIG. 1, right), uses the refractive and scattering properties of light to achieve concentration. The main advantage of luminescent concentrators is that there are no moving parts in the system. There are several disadvantages, however, including low concentration ratios (factor of 10-40 as compared to more than 100 for dual-axis tracking), low conversion efficiency, and low geometric efficiency.